This protocol will compare the effects of a selective dopamine D1 agonist (ABT-431) to intravenous levodopa in Parkinson's patients with dyskinesia. Patients with Parkinson's disease, a fluctuating response to levodopa and dyskinesia will receive single daily infusions of ABT-431 in various concentrations or intravenous levodopa on 5 consecutive days, under double-blind conditions. Parkinsonism will be scored with a clinical scring system (Unified Parkinson's Disease Rating Scale) and blood collected for pharmacokinetic.